1. Field of the Invention
The present invention relates to a toner for use in electrophotography. The present invention also relates to a method of producing toner and an image forming method using the toner.
2. Discussion of the Background
In the fields of electrophotography and electrostatic recording, latent images are generally developed with toner. Methods of developing latent image are broadly classified into methods using a two-component developer that includes a toner and a carrier, and methods using a one-component developer that includes a toner and no carrier. The former methods (hereinafter “two-component developing methods”) generally produce high grade image but have disadvantages that carrier is likely to deteriorate with time and the mixing ratio of carrier and toner is likely to fluctuate with time, which result in a shorter lifespan of the developer and unreliable image formation. In addition, the former methods do not contribute to simple maintenance and downsizing of image forming apparatus. In view of the above situation, the latter methods (hereinafter “one-component developing methods”) have attracted attention recently.
In a typical one-component developing method, a toner (i.e., a one-component developer) is fed to an electrostatic latent image formed on an electrostatic latent image bearing member, by at least one toner feeding member, to form the electrostatic latent image into a toner image. Generally, the toner feeding member feeds toner in the form of a layer. The layer of toner is preferably as thin as possible. If the layer is thick, toner particles present near the surface of the layer are sufficiently charged by a charging member while the other toner particles are not. However, if the layer is too thin, in a case in which the toner includes a release agent (such as a wax), the release agent is likely to exude from the toner with time by continuous application of mechanical stress from a toner layer forming member. As a result, the background portion of a resultant image may be soiled with toner particles (this phenomenon is hereinafter referred to as background fouling) because chargeability of the toner deteriorates. Further, the release agent may accumulate and form undesired thin film thereof on image forming members.
In one-component developing methods, the resultant image quality largely depends on the particle diameter distribution of toner. When the particle diameter distribution is wide, toner particles are selectively and successively consumed in order of particle diameter, from small to large (this phenomenon is hereinafter referred to as selective development). It may be also stated that toner particles are selectively and successively consumed in order of charge quantity, from large to small. Accordingly, the resultant image quality may deteriorate along with increase of the particle diameter of toner particles used for development. In addition, background fouling and color tone variation may be caused with time because chargeability of toner particles may deteriorate with time.
In attempting to solve the above-described problem, Japanese Patent No. (hereinafter JP) 2527473 and JP 2528511 each disclose a toner including an initial toner and a supplemental toner. The initial toner and the supplemental toner include different kinds and amounts of external additives, or the initial toner and the supplemental toner are surface-treated in different ways, intentionally, so that they have different charge quantities. However, these attempts are insufficient to prevent deterioration of image quality with time.
It is to be said that the best way to prevent deterioration of image quality is to narrow the particle diameter distribution of toner as much as possible. Various attempts have been made to narrow the particle diameter distribution of toner. For example, a pulverization method, which is one of toner production methods, has been improved to narrow the particle diameter distribution of toner, but the improvement is still insufficient. Here, in a typical pulverization method, toner components such as a binder resin and a colorant are melt-kneaded, the melt-kneaded mixture is pulverized into particles, and the particles are classified by size.
Recently, polymerization methods such as a suspension polymerization method, an emulsion aggregation method, and a polymer dissolution suspension method are also widely employed as toner production methods, as described in JP-A 07-152202 and JP-A 2007-212905, for example. Polymerization methods generally have an advantage in producing toner with a narrow particle diameter distribution compared to pulverization methods. However, polymerization methods are still insufficient to prevent selective development.
JP 3786037 discloses a toner production method in which microdroplets of fluid raw materials are formed using piezoelectric pulse and then dried into toner particles. JP3952817 discloses a toner production method in which microdroplets of fluid raw materials are formed using thermal expansion within a nozzle and then dried into toner particles. JP 3786035 discloses a toner production method in which microdroplets of fluid raw materials are formed using an acoustic lens and then dried into toner particles. However, these methods have poor productivity because the number of droplets discharged from a nozzle per unit time is small. In addition, it may be difficult to prevent coalescence of droplets, which results in a broad particle diameter distribution of the resultant particles.
Another approach involves a toner production method in which microdroplets of raw materials are formed using film vibration or liquid vibration, and then the microdroplets are discharged from a nozzle while riding on rotation feeding airflow. In this case, coalescence of droplets may be prevented, however, the resultant particle diameter distribution may not be narrow to solve the problem of selective development.